1. Field of the invention:
The present invention relates to a telephoto lens system and, more particularly, to a compact and high-performance telephoto lens system with a field angle about 12.degree. to 14.degree. and aperture ratio about F/3.5 of which variation of aberrations to be caused is small when the lens system is focused from an object at the infinite distance onto an object at a short distance.
2. Description of the prior art:
Though zoom lens systems are becoming remarkably popular in these days, there is also an increasing demand for photographic lens systems of which the focal length is not variable.
As a recent tendency regarding telephoto lens systems, there is a demand for a telephoto lens system with a large aperture ratio and high performance. Generally, for large-sized lens systems, the inner focusing method to focus the lens system by moving a lens component, which is arranged in the inner part of the lens system, along the optical axis is adopted from the view point of convenience in operation. However, the inner focusing method has disadvantages as described below. That is, when the inner focusing method is adopted, spherical aberration of the lens system becomes undercorrected when it is focused on an object at a short distance in spite of the fact that the amount of lens component movement at that time is small. As a result, the quality of image in the central portion of image becomes unfavourable and, moreover, the contrast of image decreases due to variation of chromatic aberration. The above-mentioned disadvantages become more conspicuous when the field angle of the lens system is made larger.
On the other hand, U.S. Pat. No. 3,942,875 and U.S. Pat. No. 3,942,876 (Japanese published unexamined patent application No. 38138/73) disclose so-called semi-telephoto lens systems with field angles about 18.degree. to 27.degree. which can be focused up to an object at a comparatively short distance and, at the same time, of which variation of aberrations is small.
In the above-mentioned patents, telephoto lens systems with field angles about 12.degree. are disclosed as embodiments. However, in case of said known telephoto lens systems, curvature of field and chromatic aberration which have serious influence on the performance of telephoto lens systems are not corrected satisfactorily.
Besides, generally known telephoto lens systems are designed on the basis of the state that the lens system is focused on an object at the infinite distance and are arranged to be focused by moving the lens system as a whole along the optical axis. When the above-mentioned focusing method is adopted, variation of spherical aberration becomes comparatively small but astigmatism becomes considerably undercorrected. It is difficult to correct the above-mentioned offaxial aberration. To correct said offaxial aberration, it is known to adopt so-called floating method which is often adopted for wide-angle lens systems in order to prevent variation of aberrations to be caused at the time of focusing, i.e., the method to prevent said variation of aberrations by varying some of airspaces in the lens system at the time of focusing, for example, the airspace where the stop is arranged. However, the floating method has disadvantages that the lens moving mechanism becomes complicated and the cost of production becomes high.